Free-space optical communication systems are capable of transmitting data at very high data rates over long distances. Acquisition schemes and precise beam pointing and tracking capabilities are required to communicate between moving platforms (e.g., airborne, space, and ground vehicles). Particularly with airborne platforms, where movement of aircraft can be rapid and unpredictable, it is critical that the pointing and tracking scheme provide accurate guidance for directing the data laser beams.
In two-way communications between optical communication terminals, the angular direction of a far-end terminal can be acquired and tracked by determining the angle of arrival of a signal transmitted by the far-end terminal. A dedicated beacon signal that is distinct from the data signal conveying data can be transmitted for the purpose of determining angular direction. One option is to use a single beacon signal with a fixed beam divergence. Another option is to switch the beacon laser energy between two sets of optics that sequentially produce a wide beamwidth beacon signal and a narrow beamwidth beacon signal.
Generally, a wide beamwidth signal is advantageous for initial acquisition due to the higher likelihood of detection within a given angular search region. Further, once a communication link has been established between terminals, a beacon signal with a wide beamwidth is less likely to lose track of a far-end terminal when the relative angular direction of the terminals is changing rapidly, since either terminal can fairly quickly move out of a narrower beam. Nevertheless, a narrower beamwidth signal is desirable for ongoing tracking due to its higher pointing accuracy and greater signal strength.
The disadvantage of using a single, fixed-divergence beacon signal is that the beamwidth must be selected to make a tradeoff between the aforementioned acquisition benefits of a wide beamwidth signal and the tracking benefits of a narrow beamwidth signal. For a beacon system that switches between wide and narrow beamwidths, there is likely a period of time during the switch over between the two beacon signals when there is no beacon signal being emitted, which can result in disruptions during acquisition and tracking. Accordingly, there remains a need for improved beacon systems for supporting free-space optical communications.